1. Field of the Invention
The present invention relates to a magnetoresistive element which can be used as, for example, a magnetic sensor or a magnetic memory device, and more particularly to a magnetoresistive element which achieves a change in the magnetic resistance by permitting an electric current to flow in the direction which crosses the plane of the multilayer films constituting the magnetoresistive element, and a magnetic memory unit.
2. Description of Related Art
In accordance with the rapid spreading of information communication appliances, especially personal small appliances, such as portable terminals, with respect to devices constituting these appliances, such as memory and logic, there are increasing demands of further improvement of the performance, e.g., an increase of the degree of integration, an increase of the operation speed, and lowering of the electric power needed. Particularly, an increase of the density and capacity of a nonvolatile memory is becoming more important as a technique of a substitute for a hard disc and an optical disc, which are essentially impossible to be downsized due to existence of the moving part.
As examples of nonvolatile memories, there can be mentioned a flash memory using a semiconductor and a ferroelectric random access memory (FRAM) using a ferroelectrics. However, the flash memory has a problem in that the write time is as long as a time of microsecond order. On the other hand, in the FRAM, a problem in that the rewritable ability is poor has been pointed out.
As a nonvolatile memory free of these problems, a magnetic memory device called magnetic random access memory (hereinafter, frequently referred to simply as “MRAM”) has attracted attention (see Non-patent document 1).
The MRAM has a simple structure and hence is easy to increase the degree of integration. In addition, the storage for the MRAM is made by rotation of a magnetic moment, and therefore the MRAM has a feature such that the rewritable ability is extremely excellent. Further, it is expected that the MRAM can considerably speed up in the access time, and it has already been confirmed that the MRAM can operate at an access time in the order of nanosecond.
As a magnetoresistive element constituting a memory device in the MRAM, there is a tunnel magnetoresistance (hereinafter, frequently referred to simply as “TMR”) element. The TMR element has a basic structure which is a ferromagnetic layer/tunnel barrier layer/ferromagnetic layer laminated structure. An external magnetic field is applied to the TMR element in a state such that a predetermined electric current flows a pair of ferromagnetic layers having a tunnel barrier layer disposed therebetween, so that a magnetoresistance effect appears according to the relative angle between the magnetizations in the ferromagnetic layers. Specifically, in this case, when the magnetizations in the individual ferromagnetic layers are non-parallel, the resistance value is maximum, whereas, when they are parallel, the resistance value is minimum. Therefore, in the TMR element, by creating the above parallel and non-parallel magnetization states using an external magnetic field, a change in the resistance value is caused to achieve recording of information, so that the TMR element can function as a memory device.
Especially in a spin-valve TMR element, one of the pair of ferromagnetic layers is disposed so that it is adjacent to an antiferromagnetic layer, and the ferromagnetic layer is antiferromagnetically connected to the antiferromagnetic layer to fix the direction of the magnetization in a predetermined direction, thus forming a fixed magnetization layer. Then, the other ferromagnetic layer is a magnetization unfixed layer which easily undergoes inversion of the magnetization due to an external magnetic field or the like, and this magnetization unfixed layer is used as a information recording layer in the magnetic memory unit.
When the individual spin polarizabilities of the pair of ferromagnetic layers are taken as P1 and P2, the rate of change in the resistance value in the spin-valve TMR element is represented by the following formula (1):2P1×P2/(1−P1×P2)  (1) 
The larger the spin polarizabilities P1, P2 of the ferromagnetic layers, the larger the rate of change in the resistance. With respect to the relationship between the rate of change in the resistance and the materials for the ferromagnetic layers, reports have already been made on ferromagnetic elements of the iron group, such as Fe, Co, and Ni, and metal alloys of these.
By the way, an MRAM has a basic construction including a plurality of bit write lines (so-called bit lines), a plurality of word write lines (so-called word lines) which are individually perpendicular to the plurality of bit lines, and TMR elements as magnetic memory devices disposed in portions at which the bit lines and the word lines spatially cross. Recording in the MRAM is made by selective writing for the TMR element utilizing the asteroid characteristics shown in FIG. 11 (see, for example, Patent document 1).
Specifically, a predetermined current is permitted to selectively flow the bit lines and the word lines, and an inverted external magnetic field due to synthesis of the induced magnetic fields generated in the perpendicular direction is applied to the TMR element selected, so that the direction of the magnetization in the magnetization unfixed layer, i.e., information recording layer is parallel to or non-parallel to the direction of the magnetization in the magnetization fixed layer, thus achieving recording of, for example, “0”, “1”.
As a conductive material for the bit lines and word lines in the MRAM, a wiring material for use in general semiconductor device, such as Cu, or a conductive thin film of Al or the like is used. When writing on a magnetic memory device having bit lines and word lines with a line width of 0.25 μm comprised of a general wiring material and having an inverted magnetic field Hc of, for example, 20 Oe, an electric current of about 2 mA is needed. When each of the bit lines and the word lines has a thickness of 0.25 μm which is the same as the line width, the current density is 3.2×106 A/cm2, which is close to the limit of burnout caused by electromigration.
Therefore, for maintaining the reliability of wiring, it is essential to lower the write current. In addition, from the viewpoint of preventing a problem of heat generation due to the write current and lowering the electric power consumed, it is required to lower the write current. For lowering the write current in the MRAM, it is necessary to lower the coercive force (inverted magnetic field) of the TMR element.
FIG. 11 is a so-called asteroid curve showing inverted magnetic field characteristics of the information recording layer of a TMR element constituting a memory device in an MRAM. The asteroid curve shown in FIG. 11 is an ideal asteroid curve. That is, this asteroid curve has a slenderness ratio of 1, and exhibits characteristics such that the curve form is arched.
In this asteroid curve, the ordinate is taken as the direction of difficult magnetization axis, and the abscissa is taken as the direction of easy magnetization axis, and the MRAM exhibits inverted magnetic field characteristics such that a magnetic field Hy in the direction of difficult magnetization axis generated by permitting a current to flow the word line selected and a magnetic field (auxiliary magnetic field) Hx in the direction of easy magnetization axis generated by permitting a current to flow the bit line selected are applied to the TMR element placed in a portion at which the selected word line and bit line cross, so that one ferromagnetic layer constituting the information recording layer in the TMR element undergoes inversion of the magnetization. When it is presumed that the inversion of the magnetization is caused by spin rotation, the inverted magnetic field characteristics show a curve which changes according to an asteroid curve: Hx2/3+Hy2/3=Hk2/3 (wherein Hk represents an anisotropic magnetic field) due to the synthesized current magnetic field caused by the perpendicular word and bit lines. In other words, no inversion of the magnetization occurs when Hx2/3+Hy2/3<Hk2/3, and inversion of the magnetization occurs when Hx2/3+Hy2/3>Hk2/3. 
As mentioned above, an ideal, i.e., excellent asteroid curve has a slenderness ratio of 1. When the slenderness ratio of the asteroid curve and 1 is greatly displaced, the difference in value between the inverted magnetic field and the auxiliary magnetic field required for writing is large, so that the balance between the current flowing the word line and the current flowing the bit line is poor.
Further, it is desired that the asteroid curve is arched and has a smaller curvature radius. The reason for this is as follows. When the asteroid curve is arched, the rate of change in the inverted magnetic field in respect of the auxiliary magnetic field is large, namely, the rate of change in the coercive force, i.e., inverted magnetic field from, for example, a state such that no auxiliary magnetic field is applied to a state such that a predetermined magnetic field Hsub is applied is large, and hence the sensitivity in the of the auxiliary magnetic field direction is high.
Specifically, as shown in FIG. 11, when a predetermined auxiliary magnetic field Hsub is applied, the curvature is gentle as indicated by a broken line curve As1 (shown only in the first quadrant in FIG. 11). When the curve is nearly a straight line, the inverted magnetic field Hc is reduced to Hc1 in respect of a certain auxiliary magnetic field Hsub, but the rate of change in the inverted magnetic field Hc is small, as compared to the rate of change in the solid line curve As0 having a sharp curvature, i.e., a small curvature radius, namely, the inverted magnetic field Hc0 when the auxiliary magnetic field Hsub is applied. In other words, when the asteroid curve becomes linear, the sensitivity for the auxiliary magnetic field is lowered and the auxiliary magnetic field is required to increase for obtaining the change of the inverted magnetic field, so that the write current in the MRAM is increased, leading to an increase of the electric power consumed.
In addition, from a comparison between the writable regions, i.e., so-called window areas individually defined by asteroid curves As0, As1 and a broken line “a” indicating the maximum region of the magnetic fields Hx, Hy, it is apparent that, when the asteroid curve becomes linear, the writable region is smaller. Further, when there is a lack of consistency in the asteroid characteristics of each memory device, i.e., TMR element, the asteroid curve is not comprised of one curve shown in FIG. 11 but a number of curves, and hence the width of the curve becomes substantially broad, so that the window area is further smaller and the selective writing is difficult, thus increasing the write error.
By the way, for improving the MRAM in the recording density and increasing the degree of integration of the MRAM, it is necessary to downsize the TMR element, but, when the TMR element is downsized, inversion of the magnetization is unlikely to occur, so that the inverted magnetic field Hc must be increased. Therefore, there is a dilemma that it is difficult to downsize the MRAM, namely, increase the degree of integration of the MRAM while lowering the write current.
Further, in the MRAM, when there is no consistency in the magnetic properties of TMR elements as memory devices, or there is no consistency in the magnetic properties of the same element upon repetition of the operation, the selective writing utilizing the asteroid characteristics described with reference to FIG. 11 is difficult, causing a problem in that the write error is increased.
Thus, the TMR element is needed to exhibit an ideal asteroid curve. For exhibiting an ideal asteroid curve, it is necessary that the resistance-magnetic field (hereinafter, frequently referred to as “R-H”) curve obtained by TMR measurement be free of a noise, such as Barkhausen noise, and have excellent squareness and an inverted magnetic field Hc which is stable and has consistency.
On the other hand, with respect to the reading of information in the TMR element, a state of a higher resistance value in which the magnetic moments of the information recording layer and the magnetization fixed layer having the tunnel barrier layer disposed therebetween are non-parallel, for example, “1”, and a state of a lower resistance value in which the magnetic moments are parallel, for example, “0”, are read by detecting a voltage difference, for example, at a constant bias voltage. Therefore, when the dispersion of the resistance between the elements is the same and the TMR ratio is higher, a memory device having a high speed and a high degree of integration as well as low error rate can be realized.
In addition, it has been known that the rate of change in the resistance in the TMR element has dependency on the bias voltage, and, when the bias voltage rises, the TMR ratio is reduced. Further, in the reading made by the current difference or voltage difference, in many cases, it has been known that the reading signal is maximum at a voltage Vhalf where the rate of change in the resistance is reduced by half due to the bias voltage dependency, and therefore, smaller bias voltage dependency is effective to lower the read error.
[Non-Patent Document 1]
Wang et al., IEEE Trans. Magn. 33 (1997), 4498
[Patent Document 1]
Japanese Patent Laid-Open Publication No. 10-116490
As mentioned above, in the TMR element used in the MRAM, it is necessary that both the above-mentioned write properties requirement and read properties requirement be satisfied. However, when the materials for the ferromagnetic layer in the TMR element are selected from the alloy composition comprised solely of ferromagnetic transition metal elements, such as Co, Fe, and Ni, so that the spin polarizabilities represented by P1 and P2 in formula (1) are larger, the inverted magnetic field Hc in the TMR element is generally likely to increase.
For example, when, for example, a CO75Fe25 (atm.%) alloy is used in the information recording layer, the spin polarizabilities are large and a TMR ratio as large as 40% or more can be secured, but the inverted magnetic field Hc is high. By contrast, a Ni80Fe20 (atm.%) alloy called Permalloy known as a soft magnetic material is used in the information recording layer, the inverted magnetic field Hc can be lowered, but the spin polarizabilities are small, as compared to those in the above Co75Fe25 (atm.%) alloy, and thus the TMR ratio is as low as about 33%. A Co90Fe10 (atm.%) alloy is advantageous not only in that a TMR ratio of about 37% can be obtained, but also in that the inverted magnetic field Hc can be lowered to an intermediate value between that of the Co75Fe25 (atm.%) alloy and that of the Ni80Fe20 (atm. %) alloy, but the squareness ratio of the R-H curve is poor, so that asteroid characteristics enabling writing cannot be obtained. In addition, a problem arises in that the inverted magnetic field in the information recording layer in each element is not stabilized.